In recent years there has been much interest in fabrication of articles of complex shape from silicon carbide. One particular area of interest has been the forming of complex shaped articles for gas turbine engine application from silicon carbide as this material is capable of withstanding temperatures substantially higher than the temperatures which can be withstood by present day super alloys used in gas turbine engines. When such turbine engines are operated at higher temperatures, such as the temperatures which can be used with silicon carbide components in a gas turbine engine, they become much more efficient by giving a greater amount of power for the same fuel consumption.
In attempting to form silicon carbide articles of complex shape, injection molding processes have been developed. In general, these molding processes are carried out by mixing silicon carbide particles, and optionally graphite particles, with a predetermined amount of a thermosetting binder. The article is formed in an injection molding operation, removed from the mold and subjected to heat in the absence of oxygen to reduce the thermosetting binder to carbon. The article is silicided to transform the carbon and any graphite present to silicon carbide thereby to produce a finished article of silicon carbide. A process for producing such an article is disclosed in U.S. Patent Application Ser. No. 389,770 filed Aug. 20, 1973 in the name of Douglas R. Fitchmun and assigned to the assignee of this application, which application is hereby incorporated by reference.
We have found that the prior art processes of siliciding an injection molded article containing silicon carbide and a thermosetting binder had some drawbacks. In particular, the prior art processes were slow, generally did not produce an article which was fully silicided, and were difficult to perform on a body having any substantial thickness within a reasonable period of time. We have recovered three principal reasons why the prior art siliciding processes had such difficulty.
One principal difficulty is that almost all commercially available silicon carbide powder has some silicon dioxide contained therein. This silicon dioxide is not wetted by molten silicon metal thus making a siliciding operation on an article containing this material difficult.
Another reason that the prior art processes had some difficulty in achieving a fully dense silicon carbide article is that the prior art process did not try to adjust the total amount of carbon in the article after both its formation and heating to break down the thermosetting binder into carbon. In other words, the prior art did not recognize that if more carbon was present in the article after pyrolyzing thereof than pore volume available for growth of new silicon carbide, when silicon reacted with the carbon to form silicon carbide, the result would be that the reaction would close off the pores and carbon remaining in the interior of the article would not be reached by the silicon. In such a case the surface of the article would be formed of substantially pure silicon carbide and the interior volume would be a mixture of the original silicon carbide particles and unreacted carbon.
Still another difficulty found in the prior art processes is that the prior art processes did not control the onset of the siliciding operation by application of a gaseous pressure of nitrogen to coincide with a point at which the article is be silicided was both at a proper siliciding temperature and was properly cleaned. The article is properly cleaned when all of the silicon dioxide is removed therefrom and its pore structure adjusted so that the pore volume of the article is sufficient to permit penetration of the article with a reactable form of silicon and a reaction of that silicon with all available carbon present in the article.